Understanding the pathology of vascular cognitive impairment.

The prevalence, morphology and pathogenesis of vascular dementia (VaD), recently termed vascular cognitive impairment (VCI), and of mixed dementia (Alzheimer disease+vascular encephalopathy) are a matter of discussion and no validated neuropathologic criteria for these disorders are currently available. In Western memory clinic-based series, VaD/CVI is suggested in 8-10% of cognitively impaired elderly; its prevalence in autopsy series ranges from 0.03% to 58% (mean 5-15%). Fairly unusual as an isolated nosological entity, CVI appears to correlate with focal, multifocal or diffuse cortical and/or subcortical microinfarcts and lacunes often affecting strategically important brain areas (thalamus, frontobasal, limbic system), hemispheric white matter and, less often, large brain areas. They result from systemic, cardiac or local large or small vessel disease. The lesion pattern in "pure" VCI with predominant multiple small (subcortical) lesions related to microangiopathies differs from that in "mixed dementia" (AD+VaD), more often associated with large infarcts, suggesting different pathogenesis. In very old subjects, selective hippocampal sclerosis may be accompanied by multiple other vascular pathologies. Minor cerebrovascular lesions (CVL), except for severe amyloid angiopathy, appear not essential for cognitive decline in full-blown AD, while both mild AD-type pathology and small vessel disease may interact synergistically in "unmasking" or promoting dementia. AD pathology is significantly less severe in the presence of cerebrovascular lesions. Further studies are needed to validate diagnostic criteria for VCI and to clarify the impact of vascular lesions on cognitive impairment.